The present invention relates to a glazed ceramic article, a metal and ceramic assembly having the glazed ceramic article, and a vacuum switch having the metal and ceramic assembly.
A vacuum switch is widely used for controlling supply of current for thereby controlling application of high voltage. The vacuum switch includes a pair of contacts disposed within an evacuated ceramic container in order to prevent generation of spark and short of discharge which may be accompanied by cutting off of supply of current for thereby attaining assured insulation. On the outer surface of the ceramic container of the vacuum switch is generally formed a glaze layer for making higher the insulator dielectric strength resistant to the short due to creeping discharge or the like. The glaze layer is also effective for smoothing the surface of the ceramic container for thereby preventing it from becoming dirty and making higher the chemical and mechanical strength.
The glaze layer on a ceramic main body is formed by applying a slurry of glaze on the surface of the ceramic main body and firing it (which firing is called glost firing). A ceramic material generally used for such a container whose insulation is important is alumina. A glaze of a high silicate glass content and of a low melting point is widely used since the glaze layer is formed on a sintered ceramic main body by glost firing at the temperature of 1000 to 1100xc2x0 C.
In the meantime, the ceramic container of the vacuum switch has a metal-ceramic joining portion for attaching thereto an arc shield or the like which is disposed within the container for shielding the contacts. Such joining portion is generally formed by soldering. In this connection, since the soldering temperature is lower than the glost firing temperature of the glaze, glost firing is first carried out for attaching the glaze layer to the ceramic container and thereafter a metallic member is soldered to the glazed ceramic container. Further, for reason of requirement with respect to an assembled or completed condition of a glazed ceramic article when it is supplied from a ceramic maker to a switch maker, e.g., for the reason of requirement that the manufacturing steps up to glost firing be allotted to the ceramic maker and the steps of soldering and onward be allotted to a switch maker, the glost firing step needs be carried out previously to the soldering step.
However, for the reason, for example, that glaze having been heretofore used has its softening temperature which is close to a soldering temperature (e.g., 780xc2x0 in case of widely used Agxe2x80x94Cu solder), there is sometimes caused an appearance defect due to surface roughening which is considered to be caused during soldering. Further, it is liable to adhere to such a glaze layer whose surface is roughened in this manner a dirt or the like containing a metallic constituent as a major constituent due to contaminants of a furnace (for example, metal, metal oxide or the like under high steam pressure), thus being causative of lowering the insulating ability. It is presumed that such a phenomenon is caused for the reason that the surface section of the glaze layer is softened a little to allow bubbles contained in the glaze layer to be actualized or appear at the surface section. Such a phenomenon is particularly liable to occur when the soldering step is carried out under a high vacuum condition of 1xc3x9710xe2x88x926 torr or less.
It is accordingly an object of the present invention to provide a glazed ceramic article whose glaze layer is higher in softening temperature as compared with a conventional glaze layer and which is hard to deteriorate the insulation ability or the like due to surface roughening and adherence of dirt at the time soldering of, for example, metallic members.
It is a further object of the present invention to provide a metal and ceramic assembly having the glazed ceramic article of the foregoing character.
It is a further object of the present invention to provide an insulator for support of a transmission line which has the glazed ceramic article of the foregoing character.
It is a further object of the present invention to provide a vacuum switch having the metallic and ceramic assembly of the foregoing character.
To accomplish the above objects, there is provided according to an aspect of the present invention a glazed ceramic article comprising a main body made of ceramic, and a glaze layer formed on an outer surface of the main body, the glaze layer being made of a glaze comprising 60 to 74% by weight of Si when the weight percentage is calculated in terms of SiO2 and 16 to 30% by weight of Al when the weight percentage is calculated in terms of Al2O3.
The above described glazed ceramic article is characterized in that the composition of the glaze layer is set so as to contain 60 to 74% by weight of a SiO2 constituent which is a major constituent of a glassy substance and 16 to 30% by weight of, i.e., a large quantity of an Al2O3 constituent (alumina constituent) which has a high melting point. As a result, the softening temperature of the glaze layer can be raised, thus making it possible to prevent deterioration of the appearance of the glaze layer due to its surface roughening at the time of soldering of a metallic member to the ceramic main body, particularly soldering in a high vacuum, for thereby preventing deterioration of the insulation ability due to adherence of dirt to the glaze layer. Further, even in case the glazed ceramic article is used without being soldered thereto a metallic member, it can effectively prevent roughening of the surface of the glaze layer when it is used in a high temperature atmosphere.
When the weight percentage content of Al when calculated in terms of or by conversion to Al2O3 (hereinafter referred to as WAl2O3 (% by weight)) becomes smaller than 16% by weight, the melting point of the glaze layer is lowered, thus allowing the effect of the present invention to become insufficient. On the other hand, when WAl2O3 exceeds 30% by weight, the glost firing temperature becomes too high, thus inevitably increasing the manufacturing cost. On the other hand, when the weight percentage content of Si when calculated in terms of SiO2 becomes smaller than 60% by weight, there may possibly occur such a case wherein the glaze layer cannot obtain a sufficiently high strength and insulation ability. Further, when WSiO2 exceeds 74% by weight, the flowability of the glaze layer becomes insufficient and it may possibly become difficult to raise the melting point of the glaze layer sufficiently. In the meantime, it is more preferable that WAl2O3 ranges from 17 to 23% by weight, and it is more preferable that WSiO2 ranges from 67 to 72% by weight.
The glaze layer of the glazed ceramic article of this invention may contain secondary constituents other than Al and Si so long as the above described effect is not deteriorated. Particularly, in order to adjust the melting point (or softening temperature) of the glaze and make higher the smoothness or flatness of the glaze layer, which is attained by applying a suitable fluidity to the glaze layer at the time of glost firing, the glaze layer may contain proper quantities of alkali metal constituents (particularly, Li, Na, K) or alkali earth metal constituents (particularly, Ca). In any event, it is desirable to adjust the composition of the glaze layer so that the melting point of the glaze is within the range from 1100 to 1400xc2x0 C., whereby to effectively prevent the surface roughening of the glaze layer and adherence of dirt to same which are otherwise caused at the time of soldering and prevent an excessive rise of the glost firing temperature.
The melting point of the glaze layer is herein defined as a liquidus temperature. The liquidus temperature of the glaze layer formed on the ceramic main body is determined by a heat analysis of a specimen of the glaze layer. Namely, the specimen is prepared by separating a glaze layer from a ceramic main body and subjected to a heat analysis such as DSC (Differential Scanning Calorimetry) and DTA (Differential Thermal Analysis). The liquidus temperature is determined from the temperature of the specimen at the end of an endothermic peak which is the last peak appearing at the time of a temperature rise in the analysis. Further, in case it is difficult to prepare sufficient specimens, the Al, Si and other cationic constituent contents (however, elements of extremely small quantities, i.e., elements of the quantities of less than 0.5 wt % are excluded) are analyzed by EPMA (Electron Probe X-ray Microanalyzer), XPS (X-ray Photoelectron Spectroscopy) or chemical analysis to obtain the compositions which are converted oxides (however, the compositions are converted to oxides having stoichiometric compositions by considering that the valence of oxygen is xe2x88x922, the valence of cations in the 1A group of the periodic table of the elements is +1, the valance of cations in the 2A group is +3, the valance of cations in the 3A group is +3, the valance of cations in the 4A group is +4, the valance of cations in the 5A group is +5, the valance of cations in the 6A group is +6, the valance of cations in the 7A group is +4, the valance of cations in the 8 group is +3, the valance of cations in the 1B group is +1, the valance of cations in the 2B group is +2, the valance of cations in the 3B group is +3, and the valance of cations in the 4B group is +4). Then, specimens of glass are prepared so as to have the compositions nearly equal to those obtained by the above analysis by mixing and melting the raw materials of oxides of the cationic constituents and thereafter rapidly cooling them. From the melting points of the glass specimens, the melting points of the glaze layers formed on the ceramic articles are estimated.
In case a metallic member is joined to a glazed ceramic main body by way of a solder layer, it is desired that the melting point of the glaze is higher than the melting point of the solder layer by 100xc2x0 C. or more.
For example, in case a metallic member made of ferrous metal (e.g., Fexe2x80x94Ni) is soldered to a main body made of alumina ceramic, an active solder containing an active metal constituent such as Ti and Zr can be used. In this instance, for the basic composition of the solder to which an active metal constituent is to be added, can be used an Agxe2x80x94Cu alloy (Agxe2x80x94Cu solder). The Agxe2x80x94Cu alloy does not form an intermetallic constituent with an active metallic constituent such as Ti and has a melting point which is not so high and has a good ability to be joined with ferrous metals, thus being quite desirable for use in the present invention.
In case a metallic member made of ferrous metal containing Ni is joined to a ceramic main body made of alumina ceramic by way of a solder layer, primary soldering for metallizing a joining surface of the ceramic main body can be carried out by using a primary solder containing one kind, two kinds or more kinds of active metal constituents selected from Ti, Zr and Hf, and thereafter secondary soldering can be carried out for secondarily soldering the metallic member to the metallized joining surface of the ceramic main body by using a secondary solder which is lower in the melting point and smaller in the active metal content than the primary solder. In this instance, as such a secondary solder can be used the above described Agxe2x80x94Cu solder. As such a Agxe2x80x94Cu alloy can be used, for example, a silver solder such as BAg-8 described in JIS (Japanese Industrial Standard) Z3261.
The glaze constituting the glaze layer is desired to comprise 80% or more by weight of WSiO2+WAl2O3 from a point of view of raising the softening temperature of the glaze layer. However, in order to prevent the melting point of the glaze from becoming excessively high and adjust the softening point of the glaze to a proper value, it is desired to add 3 to 20% by weight of alkali metal elements which is the content calculated in terms of oxide. The alkali metal elements are effective for adjusting the softening point of the glaze to a low temperature side. However, when the content is less than 3% by weight, the alkali metal cannot produce prominent effect. When the content is 20% or more by weight, the softening temperature is lowered excessively and the insulating ability of the glaze layer tends to be deteriorated. In the meantime, it is preferable to add alkali metal elements by the quantity within the range from 5 to 18% by weight when the weight percentage content is calculated in terms of oxides.
Then, when the glazed ceramic article of this invention is used for a vacuum switch and insulator which are required to have a high insulating ability, it is preferably to form the ceramic main body from alumina ceramic (e.g., alumina ceramic whose Al content is 85% or more by weight when calculated in terms of Al2O3). Further, by forming a glaze layer having an increased alumina content as described above on an alumina ceramic main body, the adhering quality of the glaze layer can be made higher, and the difference in the coefficient of linear expansion between the main body and the glaze layer is made smaller, thus being hard to cause a crack or cracks or crazing at the time of cooling after glost firing.